A coverlay is used as a protective cover film for flexible printed circuit boards, electronic components and the leadframe of an integrated circuit package. The coverlay can prevent damage from scratches, oxidation and contamination.
Coverlays desirably have certain electrical properties (e.g., dielectric strength), as well as acceptable structural and optical properties to provide visual appeal and security against unwanted visual inspection and tampering of the electronic components protected by the coverlay.
Typically, a coverlay comprises a single layer or multilayered polyimide film approximately 0.5 mil (12.5 microns) thick, with an adhesive coated on one side. For some applications, it is especially desirable to have a black, opaque coverlay.
Black, opaque polyimide films can be made by incorporating additives such as dyes, pigments, and/or carbon black into polyimide precursor solutions prior to imidization of the polyimide. Although this can provide films with the desired optical properties, other critical properties (e.g., film strength, dielectric properties) can be adversely affected by the additives.
There remains a need for an improved black polyimide film with good mechanical strength and dielectric properties for use in coverlay applications.